U.S. patent application number 14/499609 was filed with the patent office on 2015-04-02 for leaf spring and method of manufacture thereof having sections with different levels of through hardness.
The applicant listed for this patent is Hendrickson USA, LLC. Invention is credited to Brian Farrell, William Wilson.
Application Number | 20150091225 14/499609 |
Document ID | / |
Family ID | 52739338 |
Filed Date | 2015-04-02 |
United States Patent
Application |
20150091225 |
Kind Code |
A1 |
Wilson; William ; et
al. |
April 2, 2015 |
LEAF SPRING AND METHOD OF MANUFACTURE THEREOF HAVING SECTIONS WITH
DIFFERENT LEVELS OF THROUGH HARDNESS
Abstract
Leaf springs, and methods of manufacturing thereof, having first
and second sections, spaced apart along the length of said leaf
spring, said sections are through hardened and tempered to achieve,
respectively different levels of finished through hardness, are
disclosed.
Inventors: |
Wilson; William; (Downers
Grove, IL) ; Farrell; Brian; (Stratford, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hendrickson USA, LLC |
Itasca |
IL |
US |
|
|
Family ID: |
52739338 |
Appl. No.: |
14/499609 |
Filed: |
September 29, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61885375 |
Oct 1, 2013 |
|
|
|
Current U.S.
Class: |
267/47 ; 148/575;
148/580 |
Current CPC
Class: |
Y02P 10/253 20151101;
F16F 1/18 20130101; Y02P 10/25 20151101; C21D 1/42 20130101; B60G
2206/8402 20130101; B60G 11/02 20130101; B60G 2206/84 20130101;
C21D 9/02 20130101; C21D 1/18 20130101; C21D 1/25 20130101; B60G
2202/11 20130101; B60G 2206/428 20130101; C21D 2211/008 20130101;
C21D 2221/00 20130101; C21D 1/60 20130101 |
Class at
Publication: |
267/47 ; 148/580;
148/575 |
International
Class: |
B60G 11/02 20060101
B60G011/02; C21D 1/42 20060101 C21D001/42; C21D 1/60 20060101
C21D001/60; C21D 9/02 20060101 C21D009/02; F16F 1/18 20060101
F16F001/18; C21D 1/18 20060101 C21D001/18 |
Claims
1. A leaf spring having at least a first section and a second
section spaced apart along the length of said leaf spring, each of
said first and said second sections extending across an entire
cross section and along the length of said leaf spring, and said
first section is through hardened and tempered to a finished
through hardness and said second section is through hardened and
selectively tempered to a finished through hardness that is less
than the finished through hardness of said first section.
2. The leaf spring of claim 1 wherein the leaf spring is a
parabolic leaf spring, and said first section is a first arm of
said parabolic leaf spring.
3. The leaf spring of claim 1 having a seat and at least one eye,
wherein said second section is either the seat or the at least one
eye.
4. The leaf spring of claim 3 wherein said second section is an eye
that has been selectively tempered to a finished through hardness
of between 401 BHN and 444 BHN.
5. The leaf spring of claim 1 wherein the finished through hardness
of said first section is approximately 466 BHN to 510 BHN.
6. The leaf spring of claim 1 wherein said first section is a
trailing arm of a trailing arm leaf spring and has a finished
through hardness of between 375 BHN and 410 BHN.
7. The leaf spring of claim 1 wherein said first section has a
finished through hardness of between 444 BHN and 470 BHN.
8. The leaf spring of claim 1 wherein the finished through hardness
of the second section is about 79 to 95 percent of the finished
through hardness of the first section of the leaf spring.
9. The leaf spring of claim 1 having a third section spaced apart
along the length of said leaf spring from each one of said first
and said second sections and extending across the entire cross
section and along the length of said leaf spring, said third
section is through hardened and selectively tempered to a finished
through hardness that is less than the finished through hardness of
said first section.
10. The leaf spring of claim 9 wherein the finished through
hardness of the second or third sections is at least about 70
percent of the finished through hardness of the first section of
the leaf spring.
11. The leaf spring of claim 9 wherein the first section is
tempered to a finished through hardness of about 470 BHN, and the
second and third sections are tempered to a finished through
hardness of respectively, about 434 BHN and 406 BHN.
12. The leaf spring of claim 9 wherein the leaf spring is a
parabolic leaf spring, said first section comprises a seat and
first and second parabolic arms extending in opposite directions
therefrom and has a finished through hardness of 444 BHN to 495
BHN, and said second section comprises an eye positioned at the end
of said first parabolic arm, and said third section comprises an
eye positioned at the end of said second parabolic arm, and said
second and third sections have a finished through hardness of 388
BHN to 444 BHN.
13. A method of selectively tempering to a finished through
hardness one or more sections of a leaf spring having at least a
first section and a second section spaced apart along the length of
said leaf spring, each of said first and said second sections
extending across an entire cross section and along the length of
said leaf spring, after primary tempering of said first section has
commenced, said method comprising the acts of: applying localized
heat to a second section of said leaf spring at a temperature that
is above a temperature at which the leaf spring underwent primary
tempering and below austenitic transformation temperature,
maintaining said localized heat to said section for a period of 20
seconds or longer, and rapidly cooling the leaf spring from a
temperature of at least 50.degree. F. above the temperature at
which tempered martensite embrittlement can occur in the leaf
spring to a temperature less than 150.degree. F. by quenching the
leaf spring with an aqueous solution to reduce heat migration into
any section to which said localized heat was not applied, wherein a
resulting finished through hardness of said second section is lower
than a finished through hardness of said first section of the leaf
spring.
14. The method of claim 13 wherein the temperature at which
tempered martensite embrittlement can occur is 600.degree. F.
15. The method of claim 13 wherein the leaf spring is a parabolic
leaf spring wherein said first section thereof includes at least
one parabolic arm and has a finished through hardness of between
about 466 BHN and 510 BHN, and said second section thereof includes
at least one eye.
16. The method of claim 13 wherein said localized heat is applied
to the second section of the leaf spring to maintain the heated
areas of said second section at a temperature between 1000.degree.
F. and 1200.degree. F. for a period of time between 45 and 60
seconds.
17. The method of claim 13 wherein the second section of the leaf
spring includes at least one eye and said localized heat is applied
to said second section across an entire width of a 180 degree zone
across the top of said at least one eye.
18. The method of claim 13 wherein the second section of the leaf
spring comprises at least one eye and has a finished through
hardness of between about 401 BHN and 444 BHN.
19. The method of claim 13 wherein the second section of the leaf
spring comprises at least one eye and has a finished through
hardness of about 388 BHN or 406 BHN or 434 BHN.
20. The method of claim 13 comprising the additional acts of
applying and maintaining said localized heat to a third section of
the leaf spring, and said third section is an eye of the leaf
spring.
21. The method of claim 13 wherein said localized heat is applied
by electric heat induction.
22. The method of claim 13 wherein said second section is a seat of
said leaf spring and localized heat is applied to the seat by
applying localized heat to zones on top and bottom surfaces and
across the seat and within about a half inch on either side of a
center of the seat.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority to and the benefit
of U.S. Patent Application Ser. No. 61/885,375, filed Oct. 1, 2013,
all of which is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Disclosure
[0003] The present disclosure relates generally to leaf springs for
vehicle suspensions and to a method of manufacturing the same. The
disclosure presents several example embodiments that may be
utilized for particular purposes.
[0004] 2. Description of Related Art
[0005] In the past, quenched and tempered steel leaf springs for
trucks and other heavy duty vehicles have been specified with a
finished hardness, such as, between 375 BHN and 461 BHN (Brinell
hardness number). More recently, leaf springs in Europe and Japan
have been manufactured with a higher specified hardness, such as,
461 BHN to 514 BHN. These higher hardness leaf springs show an
improvement in fatigue life.
[0006] The demand for higher hardness leaf springs is reinforced at
least in part by the desire to reduce vehicle weight and in
particular, unsprung suspension system weight. The higher hardness
leaf springs allow for the use of fewer and/or thinner and lighter
leaf springs relative to more traditional, lower hardness leaf
springs referred to above. As a result, fuel economy as well as
control, performance and efficiency of the suspension system
improve. In addition, new laws require trucks and other heavy duty
vehicles to be capable of stopping in shorter distances, imposing
greater demands on a suspension system.
[0007] While higher hardness leaf springs show an improvement in
fatigue life, there has been, however, a persistent, low, but
nevertheless increased incidence in early failures, particularly of
the main leaf of a suspension system at the eyes when compared with
springs that are quenched and tempered to traditional hardness
ranges. Similar failures have also occurred at or around the center
or other bolt hole, if present, in the seat of the leaf spring.
These failures are the result of hydrogen environment assisted
cracking (HEAC), also known as hydrogen assisted cracking.
[0008] Hydrogen assisted cracking can occur in high strength steels
when three conditions are met: 1) a condition of static assembly
stress such as may occur as the result of clamping forces at a seat
or hoop stress from insertion of a bushing into an eye; 2) the
existence of a galvanic couple sufficient to charge the steel with
hydrogen; and 3) the steel involved is of sufficient strength to
trigger the mechanism of failure.
[0009] Hydrogen assisted cracking has a peculiarity in that as the
strength of the steel increases, the threshold stress required to
trigger hydrogen assisted cracking goes down, thus creating a
disadvantageous, inverse relationship.
[0010] In light of the foregoing, the current state of leaf springs
and in particular high hardness leaf springs, given the strong
correlation in steels between hardness and strength, leaves
something to be desired.
SUMMARY OF THE INVENTION
[0011] This disclosure is directed to leaf springs and methods of
manufacturing thereof. Through the use of secondary tempering
methods as disclosed herein, leaf springs can be manufactured with
a specified or high through hardness in the arms or the parabolic
or other sections of the leaf spring to provide high strength and
hardness, while having lower through hardness in sections of the
leaf spring that experience static assembly stress, such as in the
eyes and/or seat, thereby reducing the incidence of hydrogen
cracking and improving leaf spring fatigue life.
[0012] In one aspect, a leaf spring has at least a first section
and a second section, spaced apart along the length of the leaf
spring. Each of the first and second sections extend across an
entire cross section and along the length of the leaf spring. The
first section is through hardened and tempered to a finished
through hardness. The second section is through hardened and
selectively tempered to a finished through hardness that is less
than the finished through hardness of the first section of the leaf
spring.
[0013] In another aspect, a method is disclosed of selectively
tempering to a finished through hardness one or more sections of a
leaf spring after primary tempering has commenced. Localized heat
is applied to a section of the leaf spring, bringing the heated
areas within the section to a temperature that is above the
temperature at which the leaf spring undergoes primary tempering
and below austenitic transformation temperature. The localized heat
is maintained for at least twenty (20) seconds. The leaf spring is
then rapidly cooled from a temperature that is at least 50.degree.
F. and preferably at least 75.degree. F. to 100.degree. F. above
the temperature at which tempered martensite embrittlement can
occur down to a temperature that is less than about 150.degree. F.,
by quenching the leaf spring with an aqueous solution to reduce and
preferably minimize heat migration into any section to which the
localized heat was not applied. The result of this process is a
leaf spring having a finished through hardness in the selectively
tempered section that is lower than the finished through hardness
in at least one other section or in the remainder of the
spring.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In describing the preferred examples, reference is made to
the accompanying drawing figures wherein like parts have like
reference numerals.
[0015] FIG. 1 is a side elevational view of a parabolic leaf spring
having first and second opposing eyes at opposite ends and a
centrally located seat and showing areas to which localized heat
may be applied within these sections of the leaf spring when the
spring undergoes secondary tempering as disclosed herein;
[0016] FIG. 2 is a side elevational view of the leaf spring shown
in FIG. 1 and showing the areas relating to heat migration when
heat is applied to areas within sections of the leaf spring, as
shown in FIG. 1, during secondary tempering as disclosed
herein.
[0017] FIG. 3 is a chart of primary and secondary tempering data
for example embodiments.
[0018] FIG. 4 is a plot of the primary and secondary tempering data
provided in FIG. 3.
DETAILED DESCRIPTION
[0019] This disclosure presents examples of leaf springs which have
undergone secondary tempering, also referred to herein as selective
tempering or retempering, and methods of manufacturing the same.
The term "finished through hardness" shall mean the through
hardness of a section of a leaf spring that is through hardened and
then tempered and/or selectively tempered and subsequently quenched
and will exclude the hardness values of any decarburized layer,
which if present may extend, for example, to a depth of 0.1 mm to
0.25 mm below the surface of the leaf spring. The finished through
hardness of a through hardened, secondarily tempered section or
sections of a leaf spring prepared in accordance with this
disclosure can be verified by obtaining Vickers micro-hardness
hardness values in the section of interest at multiple depths (for
example, in increments of 0.05 mm to a depth of 0.5 mm, and
thereafter at depths of 0.75 mm, 1 mm, 2 mm, and 4 mm), excluding
any measurements associated with any decarburized layer, if
present.
[0020] As in the case of the first embodiment shown in FIGS. 1 and
2, a leaf spring 40 may be manufactured for example to include a
seat 44, optionally having a center hole 48, with arms 42, 46 which
in this embodiment comprise parabolic sections 53, 51, extending
from the seat 44 in opposite directions, and with respective eye
forms 54, 64 at the distal ends of the arms 42, 46.
[0021] The leaf spring 40 will have undergone initial processing
that is known to those skilled in the art of leaf spring
manufacture. Such initial processing includes cutting a blank of
suitable size from a bar of carbon steel alloy, such as for
example, SAE 5160, 6150, 8660 or 9260; DIN 51CrV4 or 52CrMoV4; JIS
SUP 9, 10 or 11; or Hendrickson type 4169 (a derivative of SAE 41
series alloys) referred to in CANMET (Canada Centre for Mineral and
Energy Technology) Publication entitled "SEM and microprobe
analysis of alloy 4169 for Hendrickson." For parabolic leaf
springs, the cut blank may be heated to about 1750.degree. F. or
1800.degree. F. degrees before the tapered profile is imparted to
the blank. If an eye form is present, such as eye forms 54 or 64,
the eye form or eye forms are rolled at about 1750.degree. F. to
1800.degree. F. The leaf spring 40 is then austenitized at
approximately 1550.degree. F. to 1675.degree. F. and quenched in
oil, polymer glycol or another suitable quenching solution to form
at least 90% martensite throughout the spring. Thereafter, the
entire leaf spring 40 undergoes primary tempering during which the
leaf spring is heated at a temperature, such as for example
800.degree. F. or more, that is maintained for an extended period
of time, typically 60 minutes, to achieve a desired through
hardness for the particular alloy steel being used.
[0022] As introduced by this disclosure, the process of secondary
tempering will begin after primary tempering has commenced. In one
example, the leaf spring may exit the primary tempering furnace
without having been quenched before secondary tempering begins.
Alternatively, the leaf spring may exit the primary tempering
furnace and be quenched, reducing the temperature of the leaf
spring to ambient temperature, before secondary tempering begins.
In either example, secondary tempering begins by applying heat to
select locations of the leaf spring 40, such as by heating to
1000.degree. F. to 1200.degree. F., as will be described in greater
detail herein. Preferably, for secondary tempering, heat may be
supplied by any suitable heat source, including for example,
electric induction heating, flame impingement, very high velocity
hot air flow, or fluidized bed reactor. If only the eyes are to be
treated, a brief immersion of the eyes in a bath of molten salt may
be used.
[0023] If eye forms 54, 64 undergo secondary tempering, such as for
example in FIGS. 1 and 2, the heated area, i.e. the area to which
heat is applied, should be limited to the complete eye forms 54,
64. In other words, the heated area should not extend into the
tapered section leading into the minimum thickness of the leaf
spring. Preferably, heat is applied to the outside face and across
the entire width of the eye over, for ease of explanation, a 180
degree area, for example, at 58 to 60 and/or at 68 to 70 shown in
FIG. 1. In another example, heat may be applied to the outer
quarter of the eye shown as a 90 degree area, for example, at 56 to
58 and/or at 66 to 68 in FIG. 1. Heat migration from the heated
area of the eye preferably should not exceed the location where the
parabolic sections 42, 46 of the leaf spring begin, shown as 55 and
65 in FIG. 2.
[0024] If a seat 44 undergoes secondary tempering, preferably the
heat source should be applied to areas on the top and bottom and
across the entire width of the seat and within about a half inch on
each side of the center or the center hole (if present) of the seat
44, as shown in FIGS. 1 at 48 to 50 and at 48 to 52. Heat migration
preferably should not extend beyond the seat.
[0025] During secondary tempering, the target surface temperature
of the leaf spring at the heated areas within sections undergoing
secondary tempering must be increased above the temperature at
which primary tempering was conducted.
[0026] In one example of this disclosure, secondary tempering
occurs immediately after the leaf spring exits the primary
tempering furnace without being quenched. The heated areas within
the sections undergoing secondary tempering preferably should be
heated to a minimum temperature of 1000.degree. F. and a maximum
temperature of 1200.degree. F., such as for example 1100.degree. F.
for a period of time of 20 seconds or longer. In this example, the
temperature of the heated areas within these sections should not
exceed 1300.degree. F. Maximum dwell time above about 880.degree.
F. is based on the maximum temperature at which retempering occurs.
The maximum temperature at the physical limit of the heat affected
areas should not exceed 880.degree. F. The temperature of the leaf
spring at a location one inch outside of the seat 44 preferably
should not exceed 810.degree. F.
[0027] After secondary tempering, the leaf spring 40 must be
quenched. The temperature of the leaf spring immediately prior to
quenching should be at least about 50.degree. F. and preferably at
least 75.degree. F. to 100.degree. F. higher than the point at
which tempered martensite embrittlement can occur. In this example,
temper embrittlement occurs at approximately 500.degree. F.,
accordingly, the temperature of the leaf spring should be at least
about 550.degree. F. and preferably at least 575.degree. F. to
600.degree. F. prior to quenching. After quenching, spring
temperature should be less than 150.degree. F., making the spring
cool enough to handle by hand. All sections of the spring must be
cooled.
[0028] In a further example, leaf springs were conventionally
tempered at approximately 840.degree. F. for one hour to achieve a
through hardness of 470 BHN. These example springs were subjected
to secondary tempering by maintaining surface temperature of the
heated areas of the eyes at a given temperature for a period of 45
to 60 seconds. The leaf springs in this example were manufactured
from Hendrickson 4169 material but could have been made with any
suitable material, including but not limited to those materials
cited herein. The leaf spring was approximately 4 inches wide and
11/8 inches thick at the seat and approximately a half inch thick
in the eyes. Vickers micro-hardness measurements taken in the eyes,
which underwent secondary tempering at 1000.degree. F.,
1100.degree. F., and 1200.degree. F., and then quenching, yielded
hardness values of respectively, 460 HV, 430 HV and 410 HV, which
are equivalent to respectively, 434 BHN, 406 BHN and 388 BHN when
converted to Brinell hardness values using standard correlation
charts known to those skilled in the art. As a point of comparison,
direct surface hardness measurements were taken with a King Brinell
hardness tester by placing the anvil on the inside of the eye. The
measured hardness values were approximately 20 BHN lower than the
Brinell values cited above. The reason for this minor discrepancy
is believed to be the result of attempting to directly measure the
surface hardness of a curved surface. The round ball indenter of
the Brinell hardness tester left oval shaped rather than round
impressions which are normally formed when direct surface hardness
measurements of a flat surface are taken.
[0029] Primary and secondary tempering data for example springs
appears in FIG. 3 and the plot of these data appears in FIG. 4. As
shown in these Figures, the correlation of finished through
hardness (BHN) to the temperature (.degree. F.) at which tempering
is conducted is generally linear for both primary or conventional
tempering, as well as for secondary tempering. These correlations,
however, will differ in slope, as shown in FIG. 4.
[0030] When the time for secondary tempering was extended to
approximately 15 minutes, finished through hardness fell by
approximately 18 BHN from the above-cited values. Accordingly,
temperature rather than time was shown to be the dominant influence
in hardness during secondary tempering.
[0031] As also shown in FIG. 4, when the correlation for finished
through hardness to secondary tempering temperature is extrapolated
both left and right of the three data points that define this
correlation for secondary tempering, one observes that the line for
secondary tempering will intersect the line for primary tempering
at the approximate temperature and through hardness (840.degree.
F., 470 BHN) at which the retempered leaf spring underwent primary
tempering. Further, as this correlation for secondary tempering
will vary in its vertical position, rather than its slope, for a
given primary tempering temperature and hardness, one may
extrapolate from this point with the slope of the line for
secondary tempering to predict the temperature at which secondary
tempering must be performed to achieve a desired through hardness
in the sections of the leaf spring so treated.
[0032] In yet a further example of the present disclosure, a leaf
spring having a two eyes and a seat has a first section that is
through hardened and selectively tempered to a finished through
hardness of approximately 466 BHN to 510 BHN, and a second section
that is through hardened and selectively tempered to a finished
through hardness of between 401 BHN and 444 BHN. The first section
may comprise one arm, or alternatively, both arms and the seat of
the parabolic spring. The second section may respectively comprise
one eye or the seat, or alternatively, one eye of the parabolic
leaf spring. In this example, the finished through hardness of the
second section of the leaf spring may be about 79 to 95 percent of
the finished through hardness of the first section of the leaf
spring.
[0033] In yet a further example, a leaf spring that has undergone
processing in accordance with the present disclosure has a first
section that is a trailing arm with a finished through hardness of
between 375 BHN and 410 BHN, and an eye or seat that has a finished
through hardness that is less than the finished through hardness of
the first section.
[0034] In yet a further example, a leaf spring that has undergone
processing in accordance with the present disclosure has a first
section that is tempered to a finished through hardness of about
470 BHN, and second and third sections that are selectively
tempered to a finished through hardness of respectively, about 434
BHN and 406 BHN.
[0035] In yet a further example, a parabolic leaf spring that has
undergone processing in accordance with the present disclosure has
a first section that includes a seat and first and second parabolic
arms, and a second and a third section that includes respectively,
a first and second eye positioned at the end of the first and
second parabolic arms. The first section is through hardened and
tempered to a finished through hardness of 444 BHN to 495 BHN. The
second and third sections are through hardened and selectively
tempered to a through hardness of 388 BHN to 444 BHN. In this
example, the finished through hardness of the second or third
sections of the leaf spring may be at least about 70 percent of the
finished through hardness of the first section of the leaf
spring.
[0036] In light of the above discussion, the drawings and the
attached claims, it will be appreciated that leaf springs and their
manufacture in accordance with the present disclosure may be
provided in various configurations. Any variety of suitable
materials of construction, configurations, shapes and sizes for
leaf springs and their methods of manufacture may be utilized to
meet the particular needs and requirements of an end user. It will
be apparent to those skilled in the art that various modifications
can be made in the design and manufacture of such leaf springs, and
in the performance of such methods, without departing from the
scope of the attached claims, and that the claims are not limited
to the preferred embodiments illustrated.
* * * * *